The ability to study the development and function of the human immune system, as well as the pathogenesis and treatment of diseases of the immune system is difficult due to practical and ethical limitations. Animal models, such as the mouse, are only approximate models that may not be relevant to man. In addition, for many diseases affecting man, no animals models exist since the diseases only affect man. Various strategies have been used to attempt to study the human immune system in immunodeficient mice, however, it is evident that the mouse does not provide the necessary microenvironment for providing a stable, functional human immune system. Preliminary studies in XSCID dogs suggest that the canine bone marrow and thymic microenvironments, unlike those in the mouse, are capable of supporting the development of mature, circulating human B and T cells when transplanted with human fetal hematopoietic stem cells. The over-all goal of this proposal is to document the utility of the XSCID dog as a model for studying human lymphopoiesis and immune function, and to develop in vitro assays of human lymphopoiesis in a canine microenvironment for enhancing and/or accelerating the reconstitution. The specific aims are designed to test various strategies of transplanting XSCID dogs with human CD34+ or CD3+CD38- fetal liver cells that have been shown to enhance engraftment of donor stem cells in murine and human bone marrow transplants between genetically disparate individuals. These include the dose of stem cells, the presence of bone marrow (fetal liver) derived stromal cells, and pre-transplant cytoablation. Outcome measures will include phenotypic characterization of human peripheral blood mononuclear cells, in vitro human B and T cell function, production of human immunoglobulin, and evaluation of a human primary and secondary immune response. The XSCID-hu dog, due to its size, it represents an ideal experimental small animal model in which repeated blood samples can be collected in order to study the kinetics of human lymphopoiesis, response of the human immune system to infectious disease agents, and to test vaccine and other therapeutic agents in the same animal over time. The XSCID-hu dog would also be a valuable experimental model in which to test strategies for human gene therapy.